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In Raiders of the Lost Ark, it sounds like a bunch of twigs and sticks snapping and then like crumbling paper with the high end taken out, with a little bit of cinder blocks being dragged on some concrete added in. It actually is the sound of a car rolling down a gravel driveway in neutral.

Also roll a heavy ball, say a bowling ball along a hard floor. use that bass sound that subtle low tone to add body to the rock. For impacts drop a bag of dirt or ball into a pillow to get the low freq's. If it hits a tree record the leaf sound of you hitting a branch with your hand. Try clicking a few fist sized rocks together for impacts and clicks.